This invention relates to mobile telephony and, in particular, to an improved dual mode mobile telephone.
A variety of standards have been developed for digital cellular and PCS telephone systems, based on a variety of bandwidth allocation techniques. These include the European time division multiple access (TDMA)-based Global System for Mobile communication (GSM), and the Telecommunications Industry Association (TIA) IS-136 TDMA standard (also known as North American TDMA). TDMA systems allocate unique time slots to each user within a single radio-frequency channel.
Each of these systems has widespread deployment. However, a user of a cellular telephone or PCS device that complies with one standard is inoperable in a network communicating according to another standard. Thus, typically, a user would need multiple such devices to operate in each such network. Alternatively, “dual-mode” telephones have been developed, in which the telephone is useable in two networks. However, such telephones typically require a complete baseband chipset to be provided for each of the two wireless or cordless standards. This can result in an undesirably long development time for each chipset. Moreover, each such telephone requires a unique design, which can add to development costs.
Telecommunications systems and devices, such as cellular telephones, must synchronize a plurality of clock sources. For example, in a cellular telephone, a local clock source may be used for sampling, analog-to-digital conversion, digital-to-analog conversion, and the like. However, transmitting and receiving, as well as coding, may be in response to a remotely derived clock source, i.e., a clock derived from a remote base station.
During voice communication, it is important that audio data be processed at a constant rate. The audio data rate must adjust between the local and remote clock domains. Failure to do so can result in uneven data packet separation, which can adversely affect voice quality.
A buffer is often used to even out the packet separation. A buffer is a modified (asynchronous) FIFO (first in, first out) buffer in which packets leave the buffer at a predetermined, constant rate. Minimizing the amount of actual rate adjustment is important to prevent unnecessary delays. Excessive buffering delays transmission output, while buffer under- and/or over-flow causes gaps in the data.